Linear amplifier arrangement

ABSTRACT

This invention relates to high power linear amplifiers. A high power linear amplifier is disclosed for communications such as CDMA communication systems. In accordance with the invention, there is provided a power amplifier comprising a power amplifier, a feedback circuit and a control element; wherein the architecture proposed uses an envelope detector to generate a baseband signal representing the amplitude envelope of the system input RF signal. This is digitised and used to generate phase and gain correction signals. The correction signals modulate the input signal to create a pre-distorted signal: which is applied to the power amplifier for amplification. The pre-distortion is such as to cancel the AM-AM and AM-PM distortion of the power amplifier thus resulting in an amplified output of improved spectral purity. In order to achieve the very high levels of spectral purity required by 3G cellular and multi-carrier 2G cellular systems, the correction coefficients in the lookup table need to be very precisely set. The method proposed incorporates an adaptation system whereby the pre-distorter can ‘learn’ the contents of the lookup table as it operates.

FIELD OF THE INVENTION

This invention relates to high power linear amplifiers and in particularrelates to the same using digital pre-distortion.

BACKGROUND OF THE INVENTION

First and second generation cellular systems have historically usedforms of modulation which are either constant envelope (e.g. GMSK inGSM) or which result in relatively low levels of amplitude modulation.The linearity of the high power amplifiers used for such systems hastherefore not been an important technical issue; indeed, for theconstant envelope systems it is standard practice to operate theamplifiers either close to or actually in compression in order tomaximise power efficiency.

Third generation cellular systems however typically use linearspread-spectrum modulation schemes with a large amount of amplitudemodulation on the signal envelope. When passed through a high poweramplifier, the output is typically distorted in amplitude and phase bythe non-linearity of the amplifier: the amplitude and phase distortioneffects are commonly referred to as AM-AM conversion and AM-PMconversion respectively. Both distortion effects are a function only ofthe amplitude envelope of the input signal and are insensitive to theinput phase envelope.

In systems such as Code Division Multiple Access (CDMA) modulationschemes, a plurality of signals are transmitted in a communicationsystem and are amplified simultaneously. When a plurality of signals areapplied to a linear amplifier, its non-linear characteristics will tendto produce interaction between the signals being amplified and theamplifier output will contain intermodulation products. Suchintermodulation products reduce signal quality by allowing cross-talk tooccur and such spillage often falls outside a particular licensedspectrum and must be controlled. Such intermodulation distortion can bereduced by negative feedback of the distortion components,pre-distortion of the signal to be amplified to cancel the amplifiergenerated distortion, or by separating the distortion components withthe amplifier output and feeding forward the distortion component tocancel the distortion of the amplifier output signal.

There are many ways of linearising a high power amplifier: direct RFfeedback, envelope feedback, feed-forward and pre-distortion. Forcellular power amplifiers, feed-forward amplifiers are commonly used.Feed forward amplifiers are more complicated in that they require themodification of the separated distortion component in amplitude andphase to match the gain and phase shift of the amplifier on a continuousbasis and require an error amplifier which is typically similar in powerhandling to the main amplifier: this incurs a heavy penalty in RF devicecost and power efficiency.

Envelope feedback methods (polar and Cartesian) perform much better thanfeed-forward amplifiers in terms of device cost and efficiency since theRF signal linearisation processing is done before the power amplifier ona small signal. However, envelope feedback is fundamentally limited inthe correction bandwidth obtainable by the delay of the feedback loop.As systems migrate to wider band modulation (e.g. CDMA2000 and WCDMA) alinearisation technology is required which is fundamentally a widebandtechnique.

Most implementations of pre-distortion are inherently wideband, howeverthe performance achievable has been limited by the difficulty ofmatching the complex distortion characteristics of typical poweramplifier devices with simple analogue pre-distortion networks.

U.S. Pat. No. 4,700,151 (Nagata) provides a baseband (analogue ordigital) modulation system and technique which employs a look-up tablefor adaptation. U.S. Pat. No. 5,049,832 (Cavers) provides a digitalpre-distortion arrangement which reduces memory requirements to under100 complex pairs, with a resultant reduction in convergence time andremoves the need for a phase shifter or PLL in a feedback path.

OBJECT OF THE INVENTION

The present invention seeks to provide an improved linear amplifierarrangement which achieves correction over a wide bandwidth with lowersystem cost and higher efficiency than known techniques. Moreparticularly the present invention seeks to provide a linear amplifierarrangement capable of amplifying and combining a number of frequencycarriers or bearers.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, there is provided alinear power amplifier arrangement comprising a high power amplifier, apre-distortion circuit and a feedback circuit; wherein an input isoperable to receive radio frequency RF input signals to the arrangementand is connected to the power amplifier; wherein the feedback circuitcompares a sample of the power amplifier signal output with a sample ofthe input signal to provide error signals, which error signals areemployed to modify a set of look-up values; wherein the pre-distortioncircuit receives a sample of the RF input signal and gain and phaseerror signals from the feedback circuit; and wherein the pre-distortioncircuit determines gain and phase error correction signals relative tothe set of look-up values and the sample of the RF input signal, whichgain and phase error correction signals are applied to inputs of RFamplitude and phase modulators; which error correction signals aregenerated as functions of the RF input signal in such a way that themodulated delayed RF input signal on passing through the high poweramplifier emerges with reduced distortion.

In accordance with a second aspect of the invention, there is provided alinear power amplifier arrangement comprising a high power amplifier, apre-distortion circuit and a feedback circuit; wherein a radio frequency(RF) input is operable to receive RF input signals and is connected tothe power amplifier via a directional coupler, a first delay line, an RFamplitude modulator and an RF phase modulator; wherein the feedbackcircuit comprises a directional coupler operable to sample an output ofthe amplifier and provide a signal to an amplitude and phase errordetector; wherein the pre-distortion circuit comprises a coupled linefrom the input directional coupler, a power splitter, the outputs ofwhich are connected to a second delay line and an adaptivepre-distortion subsystem; wherein the second delay line is operable toprovide a signal to the amplitude and phase error detector; and whereinthe pre-distorter subsystem is operable to receive signals from thepower splitter via an RF envelope detector and signals relating to gainerror and amplitude error from the phase and amplitude error detectorand to provide a gain correction signal to a control port of theamplitude modulator; and a phase correction signal to a control port ofthe phase modulator. The adaptive pre-distorter is therefore capable ofgenerating the correction signals as functions of a tapped RF inputsignal in such a way that the modulated delayed RF input signal, onpassing through the high power amplifier, emerges with reduceddistortion.

In order to compensate for changes in the high power amplifier gain andphase distortion characteristic, for example due to temperature orchannel frequency changes the pre-distorter operates on an adaptivebasis. That is, the pre-distorter adaptively adjusts its gain and phasetransfer functions in response to residual gain error and residual phaseerror signals fed back from an error detection subsystem.

The first delay line is operable to compensate for any delay skewbetween the signal modulation and the correction signals induced byprocessing delay in the correction path and the output is delivered asthe amplified signal. The architecture proposed provides a method ofpre-distorting the input signal to a power amplifier such that the AM-AMand AM-PM distortion generated by the power amplifier is cancelled,producing an output signal with reduced spectral regrowth in adjacentchannels.

In accordance with a third aspect of the invention, there is provided alinear power amplifier arrangement comprising a high power amplifier, apre-distortion circuit and a feedback circuit; wherein an input isoperable to receive radio frequency (RF) input signals and is connectedto the power amplifier via a directional coupler, a first delay line, afirst RF amplitude modulator, a first RF phase modulator, a second RFamplitude modulator and a RF second phase modulator; wherein thefeedback circuit comprises a directional coupler operable to sample anoutput of the amplifier and provide a signal to an amplitude and phaseerror detector; baseband processing elements; and second amplitude andphase modulators; wherein the pre-distortion circuit comprises a coupledline from the input directional coupler, a power splitter, the outputsof which are connected to a second delay line and an adaptivepre-distortion subsystem; wherein the second delay line is operable toprovide a signal to the amplitude and phase error detector; and whereinthe pre-distorter subsystem is operable to receive signals from thepower splitter via an RF envelope detector and signals relating to gainerror and amplitude error from the phase and amplitude error detectorand to provide a gain correction signal to a control port of the firstamplitude modulator; and a phase correction signal to a control port ofthe first phase modulator. The adaptive pre-distorter is thereforecapable of generating the correction signals as functions of a tapped RFinput signal in such a way that the modulated delayed RF input signal,on passing through the high power amplifier, emerges with reduceddistortion.

In this third embodiment, the architecture incorporates a ‘slowfeedback’ control system which nulls out the average gain and phaseerrors in the adaptation loop.

The baseband processing elements are feedback loops operable in the gainand phase domains in order to centre the operation of the pre-distorterand allow system components of greatly reduced operating range to beused. Preferably the amplifier arrangement further comprises, in thepre-distortion sub-circuit: an input operable to receive a time-varyingoutput proportional to the varying amplitude envelope of the RF inputsignal from the envelope generator; an anti-alias filter andanalogue-to-digital converter (ADC) signal digitiser, a lookup table(LUT) to determine gain and phase correction coefficients;digital-to-analogue converters (DACs) and anti-alias filters operable toconvert these gain and phase correction coefficients to the analoguedomain; whereby the sub-circuit can produce continuous-time correctionsignals.

The architecture provides a method whereby the LUT adaptively ‘learns’the pre-distortion coefficients required. The input signal is delayedand the output signal attenuated so that the sampled signals are equalin power and any skew in the modulation envelope is resolved. Such delayand attenuation may be intrinsically associated with the circuitry, butit is preferable that dedicated circuit components are provided wherebythe power and skew between the input signal at the gain and phasedetector may be controlled.

In accordance with a further aspect of the invention, there is provideda method of operating a linear power amplifier arrangement comprising ahigh power amplifier, a pre-distortion circuit and a feedback circuit;comprising the following steps: receiving RF input signals at an inputconnected to the power amplifier via a directional coupler, a firstdelay line, an RF amplitude modulator and an RF phase modulator; in thefeedback circuit: coupling output signals from an output of theamplifier employing a directional coupler operable to sample an outputof the amplifier and to provide signals to an amplitude and phase errordetector; in the pre-distortion circuit: coupling a sample of the RFinput signal employing a directional coupler, splitting the signal via apower splitter, feeding a first output of which to an amplitude andphase error detector via a second delay line and a second output ofwhich to an adaptive pre-distortion sub-system via an RF envelopedetector; at the pre-distorter sub-system: receiving sampled RF inputsignals from the RF envelope detector and signals relating to gain errorand amplitude error from the phase and amplitude error detector andproviding a gain correction signal to a control port of the RF amplitudemodulator; and a RF phase correction signal to a control port of thephase modulator, wherein the adaptive pre-distorter is operable togenerate the correction signals as functions of a tapped RF input signalin such a way that the modulated delayed RF input signal on passingthrough the high power amplifier emerges with reduced distortion.

A preferred fashion of determining the correction signals in the abovedescribed embodiments of the above amplifier arrangements, comprises inthe pre-distortion sub-circuit, the following steps: receiving atime-varying output proportional to the varying amplitude envelope ofthe RF input signal from the envelope generator; digitising this signalby an anti-alias filter and analogue-to-digital converter (ADC),determining gain and phase correction coefficients in a lookup table(LUT); converting these gain and phase correction coefficients to theanalogue domain by digital-to-analogue converters (DACs) and anti-aliasfilters; whereby to produce continuous-time correction signals. The gaincorrection signal modifies a delayed copy of the RF input signal via anamplitude modulator and the phase correction signal modifies the resultby a phase modulator. Preferably the LUT is typically a random accessmemory, as are widely known.

In accordance with a still further aspect of the invention, there isprovided a method of operating a linear power amplifier arrangementcomprising a high power amplifier, a pre-distortion circuit and afeedback circuit; comprising the following steps: receiving inputsignals at an input connected to the power amplifier via a directionalcoupler, a first delay line, a first amplitude modulator, a first phasemodulator, a second amplitude modulator and a second phase modulator; inthe feedback circuit: coupling output signals from an output of theamplifier employing a directional coupler operable to sample an outputof the amplifier and to provide signals to an amplitude and phase errordetector and to second amplitude and phase modulators via basebandprocessing elements which baseband processing elements comprise feedbackloops operable in the gain and phase domains and which centre theoperation of the pre-distorter; in the pre-distortion circuit: couplinga sample of the input signal employing a directional coupler, splittingthe signal via a power splitter, feeding a first output of which to anamplitude and phase error detector via a second delay line and a secondoutput of which to an adaptive pre-distortion sub-system via an envelopedetector; at the pre-distorter sub-system: receiving sampled inputsignals from the envelope detector and signals relating to gain errorand amplitude error from the phase and amplitude error detector andproviding a gain correction signal to a control port of the firstamplitude modulator; and a phase correction signal to a control port ofthe first phase modulator, wherein the adaptive pre-distorter isoperable to generate the correction signals as functions of a tappedinput signal in such a way that the modulated delayed input signal onpassing through the high power amplifier emerges with reduceddistortion.

In accordance with a yet further embodiment, there is provided acellular radio base station incorporating the amplifier arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention can be more fully understood and toshow how the same may be carried into effect, reference shall now bemade, by way of example only, to the figures as shown in theaccompanying drawing sheets wherein:

FIGS. 1a-d show graphs relating to amplifier performance;

FIG. 2 shows a first known amplifier arrangement;

FIG. 3 shows a second known amplifier arrangement;

FIG. 4 shows an amplifier arrangement in accordance with a firstembodiment of the invention;

FIG. 5 shows in detail a gain and phase error detector;

FIG. 6 shows in detail the pre-distortion sub-system;

FIG. 7 shows an amplifier arrangement in accordance with a secondembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

There will now be described by way of example the best mode contemplatedby the inventors for carrying out the invention. In the followingdescription, numerous specific details are set out in order to provide acomplete understanding of the present invention. It will be apparent,however, to those skilled in the art that the present invention may beput into practice with variations of the specific.

FIGS. 1a and 1 b show the amplitude and phase distortion characteristicsof a typical class AB power amplifier. FIG. 1a gives the output signalenvelope in volts as a function of the input signal envelope in volts,showing the characteristic amplitude compression as the amplifiers nearsits saturated output power. FIG. 1b gives the phase shift through thehigh power amplifier as a function of the input signal amplitudeenvelope.

The effect of amplifier distortion on a spread-spectrum modulated signalis illustrated in FIGS. 1c and 1 d. FIG. 1c shows the power spectrum ofa 20 channel QPSK signal applied to the power amplifier input, whilstFIG. 1d shows the resulting power spectrum at the amplifier output. Thespectrum has developed sidebands, termed ‘regrowth sidebands’ which arecharacteristic of amplifier distortion. Regrowth sidebands are a systemproblem since they can potentially interfere with neighbouringcommunication channels. Specification limits on regrowth sidebands aretherefore stringently specified in most cellular communicationstandards.

FIG. 2 shows a conventional cavity combiner transmitter system. Aplurality of separate transmitters TX1, TX2 . . . TXM each generate asignal which occupies a particular radio frequency band which is thenamplified by respective high power amplifier HPA1, HPA2, . . . HPAN. Theset of N such amplified signals are combined by a cavity combiner P10for feeding to an antenna P15. A disadvantage of this is the expensivecavity combiner which typically needs to be adjustable in order tocombine the power amplifiers with low loss, and maintain this low losseven in the event of an amplifier failure.

FIG. 3 shows an alternative transmitter architecture which is enabled byuse of a linearised amplifier such as described in this document.Multiple transmitters on various channel frequencies are combined at alow signal level by a passive combiner (P20). In contrast to the cavitycombiner (P10) described previously, the passive combiner (P20) does notneed to be low loss and can therefore be an inexpensive hybrid type.Such a passive combiner also maintains high isolation between its inputports in the event of a fault on one of its inputs. The combined output,being a composite of modulated signals at different channel frequencies,is then applied to a linearised high power amplifier (P25) such as thatdescribed in this document. Due to the improved linearity performance ofthe amplifier, the multi-carrier signal is amplified without generatingunacceptable intermodulation products and is radiated from antenna (P15)via a low cost, low loss band filter (P30).

FIG. 4 shows a block diagram of a first embodiment made in accordancewith the invention. In use, an RF input signal (10) is applied to a highpower amplifier (22) via a directional coupler (12), a first delay line(14), an amplitude modulator (16) and a phase modulator (18). An outputof the amplifier (22) provides an amplified output signal (28) which issampled by a directional coupler (26). The sampled RF output from thedirectional coupler (12) is applied to a power splitter (32), theoutputs of which are connected to an envelope detector (34) and a seconddelay line (40). The output of the envelope detector is connected to anadaptive pre-distorter subsystem (70). The adaptive pre-distortersubsystem (70) generates two outputs: a gain correction signal (92)which is connected to the control port of first amplitude modulator(16); and a phase correction signal (94) which is connected to thecontrol port of phase modulator (18).

The adaptive pre-distorter (70) generates the correction signals (92,94) as functions of input (36) in such a way that the input signal,delayed by (14) anrd modulated by modulators (16, 18), on passingthrough the high power amplifier (22) emerges with lower distortion thanif no pre-distortion subsystem had been employed. The purpose of thepre-distorter gain and phase transfer functions is therefore to cancelthe gain and phase distortion produced in the power amplifier (22). Thepurpose of delay line (14) is to compensate for any delay skew betweenthe signal (10) modulation and the correction signals (92, 94) inducedby processing delay in the correction path (12, 32, 34, 70).

The error detection subsystem (60) requires as inputs a sample (42) ofthe input signal (10) and a sample (54) of the output signal (28),normalised to the same signal level and aligned in time. Output sample(54) is normalised to the same level as (42) by attenuating the coupledoutput of coupler (26) in attenuator (52); input sample (42) istime-aligned with (54) by delaying one output of power splitter (32) indelay line (40).

In order to compensate for changes in the high power amplifier (22) gainand phase distortion characteristic, for example due to temperature orchannel frequency changes the pre-distorter (70) operates on an adaptivebasis. That is, the pre-distorter (70) adaptively adjusts its gain andphase transfer functions in response to residual gain error (82) andresidual phase error (84) signals fed back from an error detectionsubsystem (60). The pre-distortion functions therefore optimallyconverge as the system operates.

FIG. 5 shows an implementation of error detection subsystem (60) as maybe used in the above described amplifier. The input signals (42) and(54) are each split by power splitters (602) and (604) respectively. Anoutput of splitter (602) is fed to envelope detector (610) and an outputof splitter (604) is fed to envelope detector (612). The envelopedetectors (610, 612) produce output voltages proportional to theamplitude envelope of signals (42) and (54) respectively. The outputvoltage of detector (610) is subtracted from the output of detector(612) by a differential amplifier (616) to produce a signal (618)proportional to the amplitude error between (42) and (54). Thedifference signal (618) is divided in analogue divider block (620) bysignal (614) being the output of envelope detector (610) to produce asignal (82) which is proportional to the gain error between (42) and(54). The implication of this is that the gain error signal (82) is ametric (only of the gain distortion (amplitude compression or expansion)in the power amplifier and is independent of the input signal envelopelevel. This can improve the stability of the amplitude adaptation loopallowing parameter μ_(g) to be set more closely for rapid conversions.

The remaining outputs of splitters (602) and (604) are fed. to a phasecomparator (630) which has two outputs (632) and (634). If the RF inputfrom splitter (602) is represented in polar form by R₁.cos(ω_(c)t+α) andthe RF input from splitter (604) is represented by R₂.cos(ω_(c)t +β)then the response of phase comparator (630) is such that output (632) isproportional to R₁.R₂.cos(β−α) and output (634) is proportional toR₁.R₂.sin(β−α). Analogue divider block (636) divides output (634) by(632) to give phase error signal (84): it should be noted that thisdivider is merely correcting for the amplitude response of thedifferential phase detector and hence performs a different role to that(620) in the gain error loop. Phase error signal (84) is then equal totan(β−α) which for (β−α) small is approximately proportional to (β−α).

Variations of the error detector (60) are possible. Depending on theperformance required, the amplitude analogue divider (620) may beomitted (although μ_(g) will need to be set to a lower value in order topreserve a loop stability), an alternative configuration of amplitudedetectors and signal processing elements may be used. Alternative typesof phase discriminator may also be used. A variation of the errordetector (60) may be implemented which generates error signals (82, 84)relating to the signs of the amplitude and phase errors only, as arecommonly employed elsewhere in the field of control systems.

The error detection block (60) may be partially or entirely replaced bydigital implementation, wherein the RF signals (42, 54) are digitisedand the error signals (82, 84) are computed by digital signal processingmeans (DSP). The feeding of these .error signals to gain blocks (724,754) into the predistorter (70) can then be performed in the digitaldomain.

FIG. 6 shows an implementation of the adaptive digital pre-distorter(70). Signal (36), being proportional to the amplitude envelope of thesystem input (10) is filtered by a low-pass anti-alias filter (702) andis digitised by (704), an analogue-to-digital converter. The m-bitoutput of the ADC is connected to the m bits of an address bus (760) viaa multiplexer (706). The address bus (760) is used to select an addressin RAM (710) a 2^(m) word phase correction random access memory (RAM)and (740), a 2_(m) word gain correction RAM.

The data bus (713) of phase correction RAM (710) is connected to (712),a latching digital-to-analogue converter (DAC). The output of the DAC(712) is filtered by a low-pass anti-alias filter (714) to give phasecorrection signal (94). In a similar arrangement, the data bus (743) ofgain correction RAM (740) is connected to latching DAC (742). The outputof DAC (742) is filtered by low-pass anti-alias filter (744) to givegain correction signal (92).

A two phase clocking scheme is implemented. On phase one of the clock,the ADC (704) samples the filtered input signal (36) and asserts thedigital result on address bus (760) via multiplexer (706) which is openfor this path on phase one of the clock. In response to the inputaddress and with the read-write input on the RAM(762) being set to‘read’, phase correction RAM (710) asserts a correction value on itsdata bus (713) which is converted to an analogue phase correction signal(94) via DAC (712) and filter (714), which in the first embodiment islow-pass and band-pass in the second embodiment, as will becomeapparent. Similarly, gain correction RAM (740) asserts a correctionvalue on its data bus (743) which is converted to an analogue gaincorrection signal (92) via DAC (742) and filter (744). In this way thegain and phase correction signals required by the system are generatedduring clock phase one.

To facilitate adaptation, the contents of address bus (760), phase RAMdata bus (713) and gain RAM data bus (743) are clocked intofirst-in-first-out (FIFO) buffers (708), (716) and (746) respectively onphase one of the clock. The length of these buffers is adjusted so as totime align the three FIFO-stored signals with the delay on the residualgain and phase error signals (82) and (84), which are delayed byanalogue delays in the rest of the system.

On phase two of the clock, the contents of the correction RAMs (710,740) are set into write mode via their read-write inputs (762, 764) andtheir contents are adapted. The residual phase error signal (84) isfiltered by anti-alias filter (720) and digitised by ADC (722). Theresult is multiplied by a fixed coefficient μ_(p) in (724)and is appliedto a subtractor (726). The subtractor forms the difference between theoutput of FIFO (716) and the output of (724). The resulting digitalsignal (727) is applied to the phase RAM data bus (713) via a 3-statebuffer (728) which passes data on phase two but isolates it on phase oneof the clock. in a similar arrangement the residual gain error signal(82) is filtered by anti-alias filter (750) and digitised by ADC (752).The result is multiplied by a fixed coefficient μ_(g) in (754) and isapplied to a subtractor (756). The subtractor forms the differencebetween the output of FIFO (746) and the output of (754). The resultingdigital signal (757) is applied to the phase RAM data bus (743) via a3-state buffer (758) which passes data on phase two but isolates it onphase one of the clock. The settings of convergence parameters μ_(p) andμ_(p) are adjusted to achieve the desired convergence speed whilstmaintaining system stability.

The action of adaptation is as follows: on phase two of the clock,multiplexer (706) switches to pass data from FIFO (708) to address bus(760). If the delay in FIFOs (708, 716) and (746) is k cycles of thepre-distorter clock then on clock phase two the address bus (760) isthereby set to select the entries in RAMs (710, 740) corresponding tothe signal envelope k clock cycles ago. At the same time, the output ofFIFO (716) corresponds to the phase correction value which was used kclock cycles ago. The output of (724) constitutes a metric of theresidual phase error left at the same instant (k clock cycles ago) whenthis phase correction was applied, assuming the FIFO delays have beenadjusted correctly. Therefore the output of subtractor (726) representsan improved estimate of the phase correction factor needed for thesignal envelope value currently asserted on address bus (760). Duringphase two of the clock the write enable control of phase correction RAM(710) is strobed, thereby writing the improved estimate (727) into thecorrect location of the RAM.

Similarly, for adaptation of the gain correction, FIFO (746) yields, onclock phase two, the gain correction factor applied k cycles previously.Subtractor (756) forms an improved estimate of the correction factorneeded for the envelope value asserted on address bus (760) bysubtracting the scaled residual gain error at the output of (754) fromthe FIFO (746) output. During phase two of the clock the write enablecontrol of gain correction RAM (740) is strobed, thereby writing theimproved estimate (757) into the correct location of the RAM.

Whilst the above figure shows a particular embodiment of the adaptivepredistorter (70), those skilled in the art will be able to implementthe same functionality in a different arrangement of hardware and/orsoftware.

The architecture provides a method whereby the LUT adaptively ‘learns’the pre-distortion coefficients required. The input and output signalsof the system are sampled. It is preferable that the input signal isdelayed and the output signal attenuated so that the sampled signals areequal in power and any skew in the modulation envelope is resolved. Suchdelay and attenuation may be intrinsically associated with thecircuitry, but it is preferable that dedicated circuit components areprovided whereby the power and skew between the input signal at the gainand phase detector may be controlled. The two signals are each splitinto two: one output being fed into a gain error detector and the otherbeing fed into a phase error detector. The gain error detector producesan output approximately proportional to the gain error between thesignals, the phase error detector produces an output approximatelyproportional to the phase error between the signals. The gain and phaseerror signals are digitised by an anti-alias filter and ADC. As thepre-distorter operates, the input envelope samples and gain and phasecorrection samples are delayed in first-in-first-out (FIFO) buffermemory to align them in timing with the sampled error signals from thegain and phase error detectors. The gain and phase error signals aremultiplied by a fixed fractional convergence parameter and aresubtracted from the delayed gain and phase correction signalsrespectively to obtain signals which are an improved estimate of whatthe correction signals at the stored signal envelope should have been.The improved gain and phase correction estimates are written back,sample by sample, into the lookup table using the delayed envelopesignal to determine the correct addresses, thereby improving theaccuracy of the pre-distortion in future.

The method of adaptation described above modifies one look-up tableentry corresponding to one quantised envelope value for each sample ofthe gain and phase error signals. Hence each entry of the look-up tableconverges independently of the others. While having a large number ofindependent table entries has the benefit of being able to adapt to thewider strain of AM-AM and AM-PM functions, it can be disadvantageouswith respect to convergence speed and estimation error.

Variations of the algorithm are possible where the use of a singlesample of the gain and phase error signals are used to modify multipleadjacent look-up table entries, using a primary knowledge of theproperties of the look-up table function, such as limits in the rate ofchange of amplifier distortion with amplitude that are known for theamplifier employed. As an illustration of the principle, one suchvariation consists of the steps of: taking an off-line copy of thecurrent gain and phase look-up tables; adapting one or more entriesaccording to the basic algorithm; filtering using a low-pass filter theoff-line gain and phase tables with respect to envelope value index;and, writing the modified tables back into the predistorter.

In the event that the gain modulator (16) has sufficient gain adjustmentrange to absorb all system tolerances as well as all gain compressioneffects, and the phase modulator (18) has full four-quadrant phaseadjustment capability, then the pre-distorter will adapt to track outchanges in gain and phase response of the high power amplifier (22) andno further control systems are needed.

The RF modulated input waveform may be represented asx_(c)(t)=Re{x(t)exp(jω_(c)t)}, where x(t) is the baseband equivalentcomplex signal. The proposed architecture performs adaptivepre-distortion of x_(c)(t) in the polar domain by the following steps:

a) The complex modulus x_(env)(t) of the input signal x(t) is obtained,typically by an envelope detector, viz:

x_(env)(t)=|x(t)|

b) The envelope signal x_(env)(t) is sampled at rate f_(s) and quantisedto m-bits to yield a sequence of samples i_(k) where any i_(k) may takethe value 0.2^(m)−1.

c) Each i_(k) is used as an index to select an entry from a 2^(m)-entrylookup table (LUT) which stores two output coefficients for each index.One output is a gain correction sample g_(k)(i_(k)) and the other is aphase correction sample P_(k)(i_(k)).

d) The stream of selected gain correction coefficients g_(k)(i_(k)) isconverted to a continuous time analogue representation via adigital-to-analogue converter (DAC) to yield gain correction signalg(t).

e) The stream of selected phase correction coefficients p_(k)(i_(k)) isconverted to a continuous time analogue representation via adigital-to-analogue converter (DAC) to yield phase correction signalp(t).

f) The correction signals g(t) and p(t) are applied to the input signalby appropriate amplitude and phase modulators to yield pre-distortedsignal x_(p)(t) as follows:

x_(p)(t)=Re{x(t).g(t).exp(jω_(c)t+p(t))}

g) The pre-distorted input x_(p)(t) is applied to the input of the highpower amplifier, whereupon if the LUT coefficients g(i) and p(i) havebeen set correctly, the output from the power amplifier will emerge withless distortion than if no correction had been employed.

Given the power amplifier RF output waveformy_(c)(t)=Re{y(t).exp(jω_(c)t)}, where y(t) is the baseband equivalentcomplex signal, and also given a target linear RF gain G, the LUTcoefficients g(i) and p(i) are set to optimum values by the followingadaptation process:

h) A gain error signal ε_(g)(t) is derived as follows:

 ε_(g)(t)=((|y(t)|/G)−|x(t)|)/(|x(t)|)

Subject to the particular implementation of error detector (60),ε_(g)(t) may be an approximation to the above function and have arestricted linear range or relate simply to the ε_(g) of the ideal gainerror.

i) The gain error signal ε_(g)(t) is sampled at a rate f_(s) to yield asequence ε_(g,k) where ε_(g,k)=ε_(g)(k/f_(s)).

j) A phase error signal ε_(p)(t) is derived as follows:

ε_(p)(t)=angle(y(t))−angle(x(t))

Subject to the particular implementation of error detector (60),ε_(g)(t) may be an approximation to the above function and have arestricted linear range or relate simply to the ε_(g) of the ideal gainerror.

k) The phase error signal Fp(t) is sampled at a rate f, to yield asequence ε_(p,k) where ε_(p,k)=ε_(p)(k/f_(s)).

l) An improved estimate g′_(k) of each gain correction coefficient g_(k)is formed as follows:

g′_(k)=g_(k)−μ_(g).ε_(g,k)

m) An improved estimate p′_(k) of each phase correction coefficientP_(k) is formed as follows:

p′_(k)=p_(k)−μ_(p).ε_(p,k)

n) The improved estimates g′_(k) and p′_(k) are written back into lookuptable entry i_(k).

o) The sample number k is incremented by 1 and the process repeated fromstep (I).

This assumes no propagation or processing delay in the operations or inthe power amplifier. A practical implementation will require certainsignals to be delayed in order to remove timing skew, however this doesnot influence the nature of the algorithm.

FIG. 7 shows a block diagram of the second embodiment of the invention.Wherein the system includes second amplitude (20) and second phase (21)modulators and baseband processing elements (86, 87, 88, 89). These formslow feedback loops operating in the gain and phase domains in order tocentre the operation of the pre-distorter and allow system components ofgreatly reduced operating range to be used.

The operation of the slow gain feedback loop is as follows: the gainerror signal (82) is integrated by integrator (88) and amplified by gainblock (89). It is then applied to a second amplitude modulator (20)which adjusts the signal level into the second phase modulator (21) andhigh power amplifier (22). The arrangement forms a control loop withintegral action whereby the output level is adjusted to set the sampledoutput (52) at the same average envelope voltage as the sampled input(42).

Similarly, the operation of the slow phase feedback loop is as follows:the phase error signal (84) is integrated by integrator (86) andamplified by gain block (87). It is then applied to a slow phasemodulator (21) which adjusts the phase of the signal into the high poweramplifier (22). The arrangement forms a control loop with integralaction whereby the average phase of the sampled output (52) is adjustedto the same average phase as the sampled input (42). When slow loops areused, it is required that the correction signal anti-alias filters (714)and (744) have a zero at DC (i.e. are AC coupled) in order to preventthe pre-distorter adaptation and slow loop adjustments interacting.

The sampled RF output from directional coupler (12) is applied to apower splitter (32), the outputs of which are connected to an envelopedetector (34) and a delay line (40). The output of the envelope detectoris connected to an adaptive pre-distorter subsystem (70). The adaptivepre-distorter subsystem (70) generates two outputs: a gain correctionsignal (92) which is connected to the control port of first amplitudemodulator (16); and a phase correction signal (94) which is connected tothe control port of first phase modulator (18). As described in relationto the first embodiment, the pre-distorter (70) adaptively adjusts itsgain and phase transfer functions in response to residual gain error(82) and residual phase error (84) signals fed back from an errordetection subsystem (60) and the error detection subsystem (60) operatesas described above.

The ‘slow feedback’ control system nulls out the average gain and phaseerrors in the adaptation loop. The output of the gain error detectormentioned above is integrated and amplified to provide a control signalwhich modulates a gain control element between the pre-distorter and thepower amplifier itself. Similarly, the output of the phase errordetector mentioned above is integrated and amplified to provide acontrol signal which modulates a phase control element between thepre-distorter and the power amplifier. These feedback control loopsadjust to trim out the amplitude and phase errors between the two signalpaths into the gain and phase error detectors, ensuring that thesedetectors are operated at their optimum operating point. A furtherbenefit is that with the slow loops controlling the average gain andphase response of the high power amplifier, the range of gain and phaseadjustment required from the pre-distorter is greatly reduced.

Previous designs for providing pre-distorted amplification are toocomplex to be easily realisable in discrete form. The present inventionprovides a completely polar-domain design which is capable of providingpre-distortion to a standalone radio frequency power amplifier ratherthan being necessarily being incorporated into an existing DSP system.The analogue signal processing used to condition error signal andprovide input signals eliminates the need to accurately digitisewideband signals at the carrier frequency in order to drive DSPimplementations of the error feedback system and pre-distorter. Thecorrection signals from the pre-distorter are applied to the inputsignal via analogue radio frequency control elements whereby at no stageis the input signal to the power amplifier required to be in the digitaldomain. The use of slow loops can be used to stabilise the poweramplifier gain and phase response, thereby reducing the dynamic rangerequired from the pre-distorter look-up table. This is of advantage formany applications such as in the provision of high power linearamplifiers in the transmission of signals in cellular radio basestations.

Abbreviations and Definition of Terms 2G Second Generation (cellularsystem) 3G Third Generation (cellular system) AM-AM AmplitudeModulation-Amplitude Modulation (conversion) AM-PM AmplitudeModulation-Phase Modulation (conversion) ADC Analogue-to-DigitalConverter DAC Digital-to-Analogue Converter FIFO First In First Out GSMGlobal System for Mobile (Communications) GMSK Gaussian Minimum ShiftKeying LUT Lookup Table CDMA2000 A proposed third generation cellularstandard WCDMA A proposed third generation cellular standard

What is claimed is:
 1. A linear power amplifier arrangement comprising ahigh power amplifier, a pre-distortion circuit and a feedback circuit;wherein an input is operable to receive radio frequency (RF) inputsignals to the arrangement and is connected to the power amplifier;wherein the feedback circuit compares a sample of the power amplifiersignal output with a sample of the RF input signal to provide errorsignals, which error signals are employed to modify a set of look-upvalues; wherein the pre-distortion circuit receives a sample of the RFinput signal and gain a phase error signals from the feed back circuit;and wherein the pre-distortion circuit determines gain and phase errorcorrection signals relative to the set of look-up values and the sampleof the RF input signal, which gain and phase error correction signalsare applied to inputs of RF amplitude and phase modulators; which errorcorrection signals are generated as functions of the RF input signal insuch a way that the modulated delayed RF input signal on passing throughthe high power amplifier emerges with reduced distortion.
 2. A linearpower amplifier arrangement comprising a high power amplifier, apre-distortion circuit and a feedback circuit; wherein an input isoperable to receive radio frequency (RF) input signals and is connectedto the power amplifier via a directional coupler, a first delay line, anRF amplitude modulator and an RF phase modulator; wherein the feedbackcircuit comprises a directional coupler operable to sample an output ofthe amplifier and provide a signal to an amplitude and phase errordetector; wherein the pre-distortion circuit comprises a coupled linefrom the input directional coupler, a power gplitter, the outputs ofwhich are connected to a second delay line and an adaptivepre-distortion subsystem; wherein the second delay line is operable toprovide a signal to the amplitude and phase error detector; and whereinthe pre-distortion subsystem is operable to receive signals from thepower splitter via an RF envelope detector and signals relating to gainerror and amplitude error from the phase and amplitude error detectorand to provide a gain correction signal to a control port of theamplitude modulator; and a phase correction signal to a control port ofthe phase modulator, wherein the adaptive pre-distorter is operable togenerate the correction signals as functions of a tapped RF input signalin such a way that the modulated delayed RF input signal on passingthrough the high power amplifier emerges with reduced distortion.
 3. Anamplifier arrangement according to claim 2 wherein the pre-distortersub-system determines the correction signals with reference to a look uptable.
 4. An amplifier arrangement according to claim 2 wherein thefirst delay line is operable to compensate for any delay skew betweenthe signal modulation and the correction signals induced by processingdelay in the correction path.
 5. An amplifier arrangement according toclaim 2 wherein the feedback circuit further comprises an attenuatorwhich is operable to attenuate a coupled output from directional couplerand provide the signal to the amplitude and phase error detector.
 6. Anamplifier arrangement according to claim 2, further comprising, in thepre-distortion sub-circuit: an input operable to receive a time-varyingoutput proportional to the varying amplitude envelope of the RF inputsignal from the envelope generator; an anti-alias filter andanalogue-to-digital converter (ADC) signal digitiser, a lookup table(LUT) to determine gain and phase correction coefficients;digital-to-analogue converters (DACs) and anti-alias filters operable toconvert these gain and phase correction coefficients to the analoguedomain; whereby the sub-circuit can produce continuous-time correctionsignals.
 7. A linear power amplifier arrangement comprising a high poweramplifier, a pre-distortion circuit and a feedback circuit; wherein aninput is operable to receive radio frequency (RF) input signals and isconnected to the power amplifier via an input directional coupler, afirst delay line, a first RF amplitude modulator, a first RF phasemodulator, a second RF amplitude modulator and a second RF phasemodulator; wherein the feedback circuit comprises a directional coupleroperable to sample an output of the amplifier and provide a signal to anamplitude and phase error detector, baseband processing elements; andsecond amplitude and phase modulators; wherein the pre-distortioncircuit comprises a coupled line from the input directional coupler, apower splitter, the outputs of which are connected to a second delayline and an adaptive pre-distortion subsystem; wherein the second delayline is operable to provide a signal to the amplitude and phase errordetector; and wherein the pre-distortion subsystem is operable toreceive signals from the power splitter via an RF envelope detector andsignals relating to gain error and amplitude error from the phase andamplitude error detector and to provide, with reference to a look-uptable, a gain correction signal to a control port of the first amplitudemodulator; and a phase correction signal to a control port of the firstphase modulator; the baseband processing elements comprise feedbackloops operable in the gain and phase domains in order to centre theoperation of the pre-distorter; wherein the adaptive pre-distorter isoperable to generate the correction signals as functions of a tapped RFinput signal in such a way that the modulated delayed RF input signal onpassing through the high power amplifier emerges with reduceddistortion.
 8. An amplifier arrangement according to claim 7 wherein thefirst delay line is operable to compensate for any delay skew betweenthe signal modulation and the correction signals induced by processingdelay in the correction path.
 9. An amplifier arrangement according toclaim 7 wherein the feedback circuit further comprises an attenuatorwhich is operable to attenuate a coupled output from directional couplerand provide the signal to the amplitude and phase error detector.
 10. Anamplifier arrangement according to claim 7, further comprising, in thepre-distortion sub-circuit: an input operable to receive a time-varyingoutput proportional to the varying amplitude envelope of the RF inputsignal from the envelope generator; an anti-alias filter andanalogue-to-digital converter (ADC) signal digitiser, a lookup table(LUT) to determine gain and phase correction coefficients;digital-to-analogue converters (DACS) and anti-alias filters operable toconvert these gain and phase correction coefficients to the analoguedomain; whereby the sub-circuit can produce continuous-time correctionsignals.
 11. A method of operating a linear power amplifier arrangementcomprising a high power amplifier, a pre-distortion circuit and afeedback circuit; comprising the following steps: receiving RF inputsignals at an input connected to the power amplifier via a directionalcoupler, a first delay line, an RF amplitude modulator and an RF phasemodulator; in the feedback circuit: coupling output signals from anoutput of the amplifier employing a directional coupler operable tosample an output of the amplifier and to provide signals to an amplitudeand phase error detector; in the pre-distortion circuit: coupling asample of the RF input signal employing a directional coupler, splittingthe signal via a power splitter, feeding a first output of which to anamplitude and phase error detector via a second delay line and a secondoutput of which to an adaptive pre-distortion sub-system via an RFenvelope detector; at the pre-distorter sub-system: receiving sampled RFinput signals from the RF envelope detector and signals relating to gainerror and amplitude error from the phase and amplitude error detectorand providing a gain correction signal to a control port of the RFamplitude modulator; and a phase correction signal to a control port ofthe RF phase modulator; wherein the adaptive pre-distorter is operableto generate the correction signals as functions of a tapped RF inputsignal in such a way that the modulated delayed RF input signal onpassing through the high power amplifier emerges with reduceddistortion.
 12. A method of operating a linear power amplifierarrangement according to claim 11 wherein the method of providing a gaincorrection signal to a control port of the amplitude modulator; and aphase correction signal to a control port of the phase modulatorcomprises the step of referring to a look-up table.
 13. A method ofoperating an amplifier according to claim 11, further comprising, in thepre-distortion sub-circuit, the following steps: receiving atime-varying output proportional to the varying amplitude envelope ofthe RF input signal from the envelope generator; digitising this signalby an anti-alias filter and analogue-to-digital converter (ADC),determining gain and phase correction coefficients in a lookup table(LUT); converting these gain and phase correction coefficients to theanalogue domain by digital-to-analogue converters (DACs) and anti-aliasfilters; whereby to produce continuous-time correction signals.
 14. Amethod according to claim 13 wherein the LUT adaptively controls thepre-distortion coefficients required, the steps comprising: sampling theinput and output signals of the system; splitting each of the twosignals into two, feeding one output into a gain error detector and theother into a phase error detector; wherein the gain error detectorproduces an output approximately proportional to the gain error betweenthe signals, and; wherein the phase error detector produces an outputapproximately proportional to the phase error between the signalsdigitising the gain and phase error signals by an anti-alias filter andADC; whereby, as the pre-distorter operates, the input envelope samplesand gain and phase correction samples are delayed in first-in-first-out(FIFO) buffer memory to align them in timing with the sampled errorsignals from the gain and phase error detectors; multiplying the gainand phase error signals by a fixed fractional convergence parameter andsubtracting these signals from the delayed gain and phase correctionsignals respectively to obtain signals which are an improved estimate ofthe correction signals; passing, sample by sample, the improved gain andphase correction estimates back into the lookup table using the delayedenvelope signal to determine the correct addresses, thereby improvingthe accuracy of the pre-distortion for subsequent signals.
 15. A methodof operating a linear power amplifier arrangement comprising a highpower amplifier, a pre-distortion circuit and a feedback circuit;comprising the following steps: receiving input signals at an inputconnected to the power amplifier via a directional coupler, a firstdelay line, a first amplitude modulator, a first phase modulator, asecond amplitude modulator and a second phase modulator; in the feedbackcircuit: coupling output signals from an output of the amplifieremploying a directional coupler operable to sample an output of theamplifier and to provide signals to an amplitude and phase errordetector and to second amplitude and phase modulators via basebandprocessing elements which baseband processing elements comprise feedbackloops operable in the gain and phase domains and which centre theoperation of the pre-distorter; in the pre-distortion circuit: couplinga sample of the input signal employing a directional coupler, splittingthe signal via a power splitter, feeding a first output of which to anamplitude and phase error detector via a second delay line and a secondoutput of which to an adaptive pre-distortion sub-system via an envelopedetector; at the pre-distorter sub-system: receiving sampled inputsignals from the envelope detector and signals relating to gain errorand amplitude error from the phase and amplitude error detector andproviding a gain correction signal to a control port of the firstamplitude modulator; and a phase correction signal to a control port ofthe first phase modulator, wherein the adaptive pre-distorter isoperable to generate the correction signals as functions of a tappedinput signal in such a way that the modulated delayed input signal onpassing through the high power amplifier emerges with reduceddistortion.
 16. A method of operating a linear power amplifierarrangement according to claim 15 wherein the method of providing a gaincorrection signal to a control port of the fast amplitude modulator; anda phase correction signal to a control port of the fast phase modulatorcomprises the step of referring to a look-up table.
 17. A method ofoperating an amplifier according to claim 15, further comprising, in thepre-distortion sub-circuit, the following steps: receiving atime-varying output proportional to the varying amplitude envelope ofthe RF input signal from the envelope generator; digitising this signalby an anti-alias filter and analogue-to-digital converter (ADC),determining gain and phase correction coefficients in a lookup table(LUT); converting these gain and phase correction coefficients to theanalogue domain by digital-to-analogue converters (DACs) and anti-aliasfilters; whereby to produce continuous-time correction signals.
 18. Amethod according to claim 17 wherein the LUT adaptively controls thepre-distortion coefficients required, the steps comprising: sampling theinput and output signals of the system; splitting each of the twosignals into two, feeding one output into a gain error detector and theother into a phase error detector; wherein the gain error detectorproduces an output approximately proportional to the gain error betweenthe signals, and; wherein the phase error detector produces an outputapproximately proportional to the phase error between the signalsdigitising the gain and phase error signals by an anti-alias filter andADC; whereby, as the pre-distorter operates, the input envelope samplesand gain and phase correction samples are delayed in first-in-first-out(FIFO) buffer memory to align them in timing with the sampled errorsignals from the gain and phase error detectors; multiplying the gainand phase error signals by a fixed fractional convergence parameter andsubtracting these signals from the delayed gain and phase correctionsignals respectively to obtain signals which are an improved estimate ofthe correction signals; passing, sample by sample, the improved gain andphase correction estimates back into the lookup table using the delayedenvelope signal to determine the correct addresses, thereby improvingthe accuracy of the pre-distortion for subsequent signals.
 19. Acellular radio base station incorporating an amplifier arrangement inaccordance with any one of claim 1-10.
 20. A linear power amplifierarrangement comprising a high power amplifier, a pre-distortion circuitand a feedback circuit; wherein an input is operable to receive radiofrequency (RF) input signals and is connected to the power amplifier viaa directional coupler, a first delay line, an RF amplitude modulator andan RF phase modulator; wherein the feedback circuit comprises adirectional coupler operable to sample an output of the amplifier andprovide a signal to an amplitude and phase error detector; wherein thepre-distortion circuit comprises a coupled line from the inputdirectional coupler, a power splitter, the outputs of which areconnected to a second delay line and an adaptive pre-distortionsubsystem; wherein the second delay line is operable to provide a signalto the amplitude and phase error detector; and wherein the adaptivepre-distortion subsystem comprises an input operable to receive atime-varying output from the power splitter proportional to the varyingamplitude envelope of the RF input signal from an RF envelope detector,inputs operable to receive signals relating to gain error and amplitudeerror from the phase and amplitude error detector, an anti-alias filterand analogue-to-digital converter (ADC) signal digitiser, a lookup table(LUT) to determine gain and phase correction coefficients;digital-to-analogue converters (DACs) and anti-alias filters operable toconvert these gain and phase correction coefficients to the analoguedomain and to provide a continuous-time gain correction signal to acontrol port of the amplitude modulator; and a continuous-time phagecorrection signal to a control port of the phase modulator, wherein theadaptive pre-distorter subsystem is operable to generate thecontinuous-time correction signals as functions of a tapped RF inputsignal in such a way that the modulated delayed RF input signal onpassing through the high power amplifier emerges with reduceddistortion.
 21. A linear power amplifier arrangement comprising a highpower amplifier, a pre-distortion circuit and a feedback circuit;wherein an input is operable to receive radio frequency (RF) inputsignals and is connected to the power amplifier via an input directionalcoupler, a first delay line, a first RF amplitude modulator, a first RFphase modulator, a second RF amplitude modulator and a second RF phasemodulator; wherein the feedback circuit comprises a directional coupleroperable to sample an output of the amplifier and provide a signal to anamplitude and phase error detector; baseband processing elements; andsecond amplitude and phase modulators; wherein the pre-distortioncircuit comprises a coupled line from the input directional coupler, apower splitter, the outputs of which are connected to a second delayline and an adaptive pre-distortion subsystem; wherein the second delayline is operable to provide a signal to the amplitude and phase errordetector; and wherein the adaptive pre-distortion subsystem comprises aninput operable to receive a time-varying output from the power splitterproportional to the varying amplitude envelope of the RF input signalfrom an RF envelope detector, inputs operable to receive signalsrelating to gain error and amplitude error from the phase and amplitudeerror detector, an anti-alias filter and analogue-to-digital converter(ADC) signal digitiser, a lookup table (LUT) to determine gain and phasecorrection coefficients; digital-to-analogue converters (DACs) andanti-alias filters operable to convert these gain and phase correctioncoefficients to the analogue domain and to provide a continuous-timegain correction signal to a control port of the first amplitudemodulator; and a continuous-time phase correction signal to a controlport of the first phase modulator, the baseband processing elementscomprise feedback loops operable in the gain and phase domains in orderto centre the operation of the pre-distorter; wherein the adaptivepre-distorter subsystem is operable to generate the continuous-timecorrection signals as functions of a tapped RF input signal in such away that the modulated delayed RF input signal on passing through thehigh power amplifier emerges with reduced distortion.
 22. A method ofoperating a linear power amplifier arrangement comprising a high poweramplifier, a pre-distortion circuit and a feedback circuit; comprisingthe following steps: receiving RF input signals at an input connected tothe power amplifier via a directional coupler, a first delay line, an RFamplitude modulator and an RF phase modulator; in the feedback circuit:coupling output signals from an output of the amplifier employing adirectional coupler operable to sample an output of the amplifier and toprovide signals to an amplitude and phase error detector; in thepre-distortion circuit: coupling a sample of the RF input signalemploying a directional coupler, splitting the signal via a powersplitter, feeding a first output of which to an amplitude and phaseerror detector via a second delay line and a second output of which toan adaptive pre-distortion sub-system via an RF envelope detector; atthe adaptive pre-distorter sub-system: receiving a time-varying sampledRF signal proportional to the varying amplitude envelope of the RF inputsignal from the envelope detector, digitising this signal by ananti-alias filter and analogue-to-digital converter (ADC), receivingsignals relating to gain error and amplitude error from the phase andamplitude error detector, determining gain and phase correctioncoefficients in a lookup table (LUT); converting these gain and phasecorrection coefficients to the analogue domain by digital-to-analogueconverters (DACs) and anti-alias filters, and providing acontinuous-time gain correction signal to a control port of the RFamplitude modulator; and a continuous-time phase correction signal to acontrol port of the RF phase modulator; wherein the adaptivepre-distorter subsystem is operable to generate the continuous-timecorrection signals as functions of a tapped RF input signal in such away that the modulated delayed RF input signal on passing through thehigh power amplifier emerges with reduced distortion.
 23. A methodaccording to claim 22 wherein the LUT adaptively controls thepre-distortion coefficients required, the steps comprising: sampling theinput and output signals of the system; splitting each of the twosignals into two, feeding one output into a gain error detector and theother into a phase error detector; wherein the gain error detectorproduces an output approximately proportional to the gain error betweenthe signals, and; wherein the phase error detector produces an outputapproximately proportional to the phase error between the signals;digitising the gain and phase error signals by an anti-alias filter andADC; whereby, as the pre-distorter operates, the input envelope samplesand gain and phase correction samples are delayed in first-in-first-out(FIFO) buffer memory to align them in timing with the sampled errorsignals from the gain and phase error detectors; multiplying the gainand phase error signals by a fixed fractional convergence parameter andsubtracting these signals from the delayed gain and phase correctionsignals respectively to obtain signals which are an improved estimate ofthe correction signals; passing, sample by sample, the improved gain andphase correction estimates back into the lookup table using the delayedenvelope signal to determine the correct addresses, thereby improvingthe accuracy of the pre-distortion for subsequent signals.